1. Field of the Invention
The present invention relates to a liquid crystal display device and in particular relates to a liquid crystal display device provided with an alignment film whereby satisfactory alignment of the liquid crystal molecules can be achieved.
2. Description of the Related Art
Reflective type liquid crystal display devices are employed as display devices for, for example, watches, desk-top calculators and portable equipment, since they can be operated with very low power. A conventional reflective type liquid crystal display device whereby a bright image can be displayed is disclosed in Japanese Patent Publication No. 52843/1991 and Early Japanese Patent Publication No. 271233/1988. Such liquid crystal display devices are known as polymer diffusion type (PDLC type) liquid crystal display devices and are constructed by sealing a liquid crystal layer constituted by a liquid crystal material consisting of a mixture of polymer resin and a nematic liquid crystal between two substrates. It is arranged that one of the two refractive indices produced by the birefringence possessed by the nematic liquid crystal material coincides with the refractive index of the polymer resin.
When the refractive index of the liquid crystal material when voltage is applied and the refractive index of the polymer resin are made practically the same, in the condition where no voltage is applied between the substrates of the liquid crystal display device, refraction and/or reflection of input light occurs at the interface between the liquid crystal material and the polymer resin material, causing scattering of the input light. This is therefore observed from the outside as cloudy white (white colour).
In contrast, in the condition where voltage is applied between the substrates of the liquid crystal display device, since the refractive index of the liquid crystal material and the refractive index of the polymer resin material are practically the same, no refraction and/or reflection of the incident light occurs, and the incident light is therefore transmitted and is absorbed by the optical absorption layer. This is therefore observed from the outside as black colour. However, a prior art liquid crystal display device as described above suffers from the following inconveniences.
(1) High Drive Voltage
In the prior art liquid crystal display device, voltage division of the applied voltage occurs between the liquid crystal material, which is of comparatively high dielectric constant at 10.about.15 and the polymer resin material which is of comparatively low dielectric constant of about 3. Since the result of the voltage division is that more voltage is applied where the dielectric constant is lower, only a relatively low voltage is applied to the liquid crystal material. In image display, control is required whereby the long axes of the molecules of the liquid crystal are changed over to the perpendicular direction or horizontal direction of the substrates. If therefore sufficient voltage is applied to control the changeover of the liquid crystal molecules, a high drive voltage of at least about 10 volts must be applied to the liquid crystal display device as a whole. If this is compared with the three volts or so of drive power of a twisted nematic type liquid crystal display device, it can be seen how much power consumption is required by the conventional liquid crystal display device.
(2) Low Degree of Backwards Scattering
The degree of backwards scattering means the ratio of scattered light returning to the observer as a result of the input light being subjected to optical scattering at the liquid crystal layer in the optical scattering condition. Increasing this value increases the brightness of the display.
In conventional liquid crystal display devices, this degree of backwards scattering is at most about 20%. The low brightness of conventional liquid crystal display devices can be seen from the fact that the degree of backwards scattering of the white portions of a newspaper page is about 70%. In order to raise the degree of backwards scattering, the thickness of the liquid crystal layer may be increased or the density of the polymer resin may be raised, but, if these measures are adopted, the drive voltage becomes even higher. To obtain a satisfactory backwards scattering degree, a drive voltage of at least 30 volts is needed, so power consumption is even further increased. The above inconveniences can therefore be eliminated if a reflective liquid crystal display device could be constructed by a construction other than the combined construction of liquid crystal and resin material or if the degree of backwards scattering could be raised without increasing the thickness of the liquid crystal layer and/or the density of the polymer resin.
Having regarding to the above circumstances, according to the present invention a reflective liquid crystal display device is implemented by providing an alignment control film that is different from conventionally.
Specifically, a first task of the present invention is to provide a reflective liquid crystal display device of comparatively low drive voltage without employing a construction combining an liquid crystal material and polymer resin material, and different from the conventional polymer dispersion type liquid crystal display device. Also, a second task of the present invention is to provide a reflective liquid crystal display device wherein there is no attenuation of light such as results from the combined construction described above and wherein the optical scattering performance, optical transmissivity and contrast ratio are improved from conventionally.
Also, a third task of the present invention is to provide a reflective liquid crystal display device wherein the degree of backwards scattering is raised and that is brighter than conventionally without increasing the thickness of the liquid crystal layer and/or increasing the density of the polymer resin material in a combined structure as described above i.e. without raising the drive voltage.
A fourth task of the present invention is to provide a liquid crystal display device and method of manufacturing it provided with an alignment control film that is more resistant to wear than the alignment film employed conventionally and that can be manufactured easily and at low cost.